Final Report Summary - GEOSOX (GEOSOX: Geochronology of secondary oxide minerals in bedrock and fluid flow and deformation histories)
Our work focused primarily on mineralogic and textural characterization by scanning-electron microscopy (SEM), developing dating methods by several techniques, and measurement of oxygen- isotope compositions by secondary-ion mass spectrometry (ion probe). This was done on a variety of secondary oxide samples from bedrock in Arizona and Colorado. A larger number of samples from a wider variety of locations were dated by (U-Th)/He methods.
Figure 1 shows an example of a typical secondary Fe-oxide (and Mn-oxide) layer coating a fracture surface of a 1.6-Ga rhyolite in Arizona. The SEM and ion-probe analyses were performed at CRPG. This representative sample shows the polycrystalline and polymineralic textures common in these types of specimens, and examples of the ion-probe spots we use to measure O-isotope compositions. This particular specimen yielded (U-Th)/He dates of 1.0-1.1 Ga on the Fe-oxide (hematite) and 20-30 Ma on the Mn-oxide. The hematite showed a wider range of oxygen isotope compositions compared with the Mn-oxide (δ18O of about -4 to -2 and -1, respectively).
Our study included attempts to develop a new and relatively rapid analytical approach to dating secondary oxides that involves in-situ excimer laser ablation for He extraction and measurement. Unfortunately, this method proved inefficient because of poor laser-oxide coupling. After two weeks of mass spectrometer time, this approach was abandoned. However, routine conventional (U-Th)/He dating at UA provided a suitable approach by which we successfully dated a large number of specimens.
In all, we successfully dated dozens of aliquots from about six secondary oxide samples from veins, fractures, and faults by the (U-Th)/He method and combined these with oxygen-isotope analyses determined by ion-probe. We determined that most secondary hematite has δ18O of about -5 to 0‰. In some cases, (U-Th)/He dates are quite reproducible and match expectations based regional geologic constraints. For example, hematite in 14-16-Ma Basin and Range normal faults yield (U- Th)/He ages matching those of faulting and dates from other isotopic systems, and hematite from faults in crystalline bedrock in several other regions yields dates of 1.0-1.1 Ga, corresponding to major tectonic and magmatic episodes in the Arizona. In some cases, however, secondary oxide dates are variable show poor reproducibility. Our analyses suggest that this reflects a wide range crystal sizes, chemistry, and thermal histories involving slow cooling.